Multichannel transmission system



Sept. 23, 1952 D. B. HARRIS 2,611,825

MULTICI-IANNEL TRANSMISSION SYSTEM Filed April 28, 1948 7 Sheets-Sheet 1 FROM omoc LOW AUDIO F,,(I)cos 10 I PASS LEAMI? uum .JER OUTPUT I? g 2 (n I 2 cos 2 g 0 FROM BEBMNOTJULATING FIGURE A CARRIER SOURCE DEMODULATION OF COSINE CARRIER LOW AUDIO FROM 2%(1) sm um DEMOD PASS LEAMF. 'JLATCP '-'|LTER OUTPUT 3 2 5' coszw 2 71 FROM DEMODULATING w CARRIER SOURCE DEMODULATION OF. SINE CARRIER xr grg FROM fi wos um DEMOD -g g AUDIO LEAMP F (l)S|Nw I ULATOR FILTER OUTPUT 2 5 coszw kF z-(e) sm 20m FROM DE'M'OTDULATING cmmsa SOURCE FIGURE I.C DEMODULATION OF COSINE CARRIER IN PRESENCE OF SINE CARRIER. j Wm 02 FROM on LOW AUDIO PASS Ema-I um FILTER I OUTPUT F (I)coswe+F (QSINQH I '51. Occsz 5 0) swam +|-;,:)c0sw I+r-; t)smm I I cos@ w,)t i cos.(@ .o,)I

e cos. r r5 0 (away X5 5 (2) s|N(m (ADI.

W FROM DEMODULATING CARRIER SOURCE FIGURE I. D

DEMODULATION OF COSINE CARRIER IN THE PRESENCE OF SINE CARRIER ON SAME FREQUENCY AND COSINE AND SINE CARRIERS ON ANOTHER FREQUENCY.

FIGURE I. THE PRODUCT DEMODULATION -PRINCIPLE 1 EN OR.

ATTORNEYS D. B. HARRIS MULTICHANNEL TRANSMISSION SYSTEM Sept. 23, 1952 7 SheetsP-She efr 3:

Filed April 28, 1948 OUT- FIGURE 2. A

v 0 w T E A OB L RU w .L. C W E E E6 D G u m Yl U L w m U R MO EC DEMODULATING CARRIER OSCILLATOR GRID VOLTAGE d mmmmmmm mwmm T RU C F O G D AOV N R D L O T C H wN s m mm 2 EFRSR T E T E R TCAOEC m L T G M A F BCAPT INVENTUR. 5

TOR I M BY 2 f 2 ATTORNEYS FIGURE 2. PRODUCT DEMODULA Sept. 23, 1952 D. B. HARRIS MULTICHANNEL. TRANSMISSION SYSTEM Filed April 28, 1948 7 Sh ee ts- Sheet 3 INV N ZWM' ATTORNEYS Sept. 23, 1952 D. B.,, HJAR1$ MULTICHANNEL. TRANSMISSION SYSTEM Filed April 28'. 1948 7 Sheets-Sfhlegt 4 1 2d Mam ATTORNEYS Sept. 23, 1952 D. B. I-IARR s 2, 1 25 MULTICHANNEL TRANSMISSION SIYSTEM Filed April 28, 1948 7 Sheets-Sheet 5 I PASS-BAND OF CHANNEL cAR- 30 500 RIER SECTION.

--ao425- LEAMPLIFIERIW -30 275- 30400 I I 30270 30272 CHANNEL4 DRIFTZOOKC 30300 I m 3o267 30 CHANNEL 30 26 CHANNEL3 I I 30245 cARRIER BAND DRIFTZOO KC 1 30 [0o I 30 256 HANNEL2 I 30 253 O45 PASS-BAND OF I 0 25I MIXER OUTPUT 30 250- 30000 SELECTIVITY 30 248 HANNEU DETAIL OF CHANNEL cARRIER BAND DRIFT 200 KC 29 74-4 CONTROL 29 72B cARRIER BAND M 29 700 -29 744 LSJIBDEEBIAND PASS-BAND OF ---29 625 CARRER 29 736 CARRIER 29 500 FIER ll4 LOCATION OF CHANNEL CARRIER AND 29 728 -fg gfi CONTROL CARRIER BAND IN LE SPECTRUM.

FIGURE 5.

FRE UENCY ALLOCATION IN LE STAGES FOR FREQUENCY CONVERSION METHOD APPLIED TO MICRO-WAVE TRANSMISSION.

TAI F CONTROL CARRI BAN INVENTOR.

. BY I mMM M Arrggzggs Sept. 23, 1952 D. a HARRIS 2,611,825

MULTICHANNEL TRANSMISSION SYSTEM Filed April 28, 1948 7 Sheets-Sheet 7 FIGURE 7. DETAILED SCHEMATIC CIRCUIT OF SYNCHRONIZED OSCILLATO R- DEMO- DULATORS USED IN FIGURE 6.

TUBE A FROM BAND AMPLIFIER OUT PUT. TUBE B 213 2' 235- SINE CHANNEL OUTPUT Q gVENTOR.

4 MW ATTORNEYS Patented Sept. 23, 1952 UNITED STATES PATENT OFFICE; I 2,611,825

MULT-ICHANNEL;TRANSMISIS-ION- SYSTEM:

.D.onald.B.iHarris, Cedar Rapids, Iowa Application April 28, 1948; Serial No. 2?;667?

. 6C1aims.v -l invention :relates to:multi-channel transmissionisystems :ofthextypezadapted to transmit a-ppluralityof. messages simultaneously over a common :facility such as atelephone line; or a coaxialxcable; or'through a. common transmis sionymedium, as by means of .a radio wave.-

:In: the case of'wire lines; present frequencydivision multiplex carrier systemsgshare the disadvantage: that although in manycases the cheapest available instrumentality for effectingcommunication between two points; they are nevertheless" unduly" costly in comparison with other-electronic systems' containing a' comparable. amount of apparatusizsuch as, 'for example;

radio: broadcast receivers. This excessive costthe particular channel frequency but. also. the particular sideband required-before impressing: the signal onthe demodulator.. As; these-:filters operate: atzrelatively low frequencies; they; con+ tain components: of comparatively large.- size:

which-are bulky-"and expensive; and in 'order efieictivelyto utilize the available frequency "space" of-nthe transmissionvmedium they must he de signed to extremely close-frequency 'andzbancb width-tolerances, adding to their: complexity and cost of. manufacture; This practice-10f pre-se lection of the. desired channel also-results; in

a requirement that the frequency deviation of.

thecarrier be held within close limits, in order.

to avoid any possible. drift. of" a. given channel.

intothe frequency space reserved for :neighboring channels. The resulting refinement. inadesign :requiredin the oscillators and other. equipment used. to-generate thecarriers also adds; to;

the 'cost;

Efforts to employ pre-selecting-carrier-' equip-- ment of this type in conjunction with radio? transmitters and receivers forythe" purpose of: producing multi-cha-nnel radio transmission :sys-- Ifpa multia tems have so farnot been successful. channel band is generated at relatively low fre quencies by meansof existing carrierrequipment.

andthis band is-used to modulate iatradior-wave whichis detected in the:receiveri-torrestorethes multiechannel. band, the cross-modulation. Jae--- sulting" from the non-linear characteristics? of the :radio system modulators and: detectors makes it .necessary to; reduce the. individual channel levels '-toa; point. where the operation @ofsthe sys tem .is not economical. If; onthemt-herwhand; anattempt is madeeto select and separate. the channels at radio or intermediate frequencies; it becomes-tmandatory to provide large frequency, separations between "the-channels; :ini order'vto allow for the unavoidabledritt of: thewoarrier frequencies. At" microwave frequenoiesgthese separationsbecome so large :as-torender the= syse tem as a -whole extremely wasteiul of frequency space: If for. example; operation at 4000 mega.- cycleswith a frequency stability of; .005 spelt-:- cent is assumed (a stringent; requirement-which hasnotvyet beenattai-nedwin practice.) itais evie dent that each carrier may be expected; to==drit ZOO-kiloeycles to eitherside. of. its nominal; ee quency. Under these :conditions;; cit-.issee that a channel bandwidth of. 40.3.,000 acycles .is. .e.-. quiredto transmit and receive each; .3000 channel. .A-system-eontaining =echannelsiwill therefore require an overall bandwidth; :of 120 x 403900: cycles=48:36-= megaeycleaa-compared with a bandwidth of .'36 'megacycle,:whichwwould be required .if. all the .frequency space :were Y utilized.

Asa matter-of fact, the-present condition of the art, other difficulties'relating,tolackmf R.-.F. selectivity "and.cross-modulationin the radio frequency stages have-"made it necessary to. separate the channels still. further, so -that in: systems now in experimental operation each channel is separated 40 megacycles from. its: nearest neighbor. A bandwidth of more than 40,000,000 cycles. is therefore required-to. trans-1 mita single channel only 3000" cycles Widflpfifld it is obvious that true multi-channelopera-- tion cannot be carried out under these condi tions.

In the present invention;. design economies-ware effected .and .full advantageof'the available frequency space is taken by eliminating the chant.- nel selecting filters; and obtaining.the-required selectivity as-an adjunct to the process' -ofi :de..- modulation. Both sidebands' of each-*channel are :generatedat thertransmitting terminal; :the carrier may orm'aynot be:suppressed;., depende' ing on the amount "of. intercliannel. crosstalk: which'can'betolerated; Inithisifashioma multi channel band is produced at relatively lowfre quenc'y; This 'band 'may' be transmitted-directly to. the receiving. terminal "over a =wire, -'cab le;' =or' coaxial line, or maybe used to=modulate*a=-'radio: frequency carrier for transmission by radio'i At'the receiving terminal, the incoming signal; containing: the entire spectrum; 'isamplified in' radio-frequency and intermediate frequency amplifiers, or in the case of wire, cable, or coaxial transmission, in a carrier frequency amplifier, without attempting to separate the channels or sidebands before amplification. If radio transmission is employed, the signal is in general heterodyned with a local oscillator signal during this process, so that the multi-channel band after amplification occupies approximately the same frequency space as it occupied in the transmitter following the original modulation process- The complex signal is then applied, in parallel, to a number of demodulators, one for each channel. These demodulators are of the linear type, so that cross-modulation between the channels is not produced in the process of demodulation. Each demodulator is supplied with a demodulating carrier identical in frequency and phase with the carrier of the channel to be demodulated, as this carrier appears at the demodulator, and is so constituted that its useful output is a linear function of the product of the instantaneous amplitude of the signal impressed on the demodulator input, and the instantaneous amplitude of the demodulating carrier. As the output signal is a linear function of the input signal, cross-modulation between channels does not occur in the demodulator, although the signal impressed on the input of each demodulator contains the carriers and side-bands of all channels in the entire spectrum. Under these conditions the output of the demodulator contains the original modulation of the particular channel to be demodulated, plus frequency components representing the otherchannels in the band but in all cases displaced in frequency upward beyond the highest modulation frequency of the channel being demodulated. These components are removed by the integrating action of a low-pass filter in the output of the demodulator, so that only the original modulation of the demodulated channel remains.

In order to assure proper conformance in frequency and phase between the channel carriers and the demodulating carriers, the demodulating carriers are maintained in step with their corresponding channel carriers, either by deriving the demodulating carriers from a control carrier transmitted from the transmitting terminal, or by injecting a portion of the channel carrier voltage into the oscillator of each channel demodulator. Any variations in phase or frequency caused by instability in the oscillators of the transmitting and receiving terminals, by the transmission medium, or by changes in the circuit constants of the equipment, are imparted equally to the channel carriers and their corresponding demodulating carriers and they remain exactly in phase at all times.

If, in the case of radio transmission, therefore, the radio frequency carrier drifts, the entire transmitted spectrum drifts with it, the various channels and the control carrier maintaining the same relative positions with respect to each other; and as the channels are not separated until after demodulation, this drift is of no consequence from the standpoint of channel selectivity. As a result, the spacing of the channels in the spectrum can be made very small, the magnitude of this spacing being principally dependent on the sharpness of the cutoif characteristics of the low pass filters inthe output of the demodulators. For example, if these filters can be designed to produce a large attenuation 500 cycles beyond the cutoff point, the spacing between channels need be only 500 cycles. The transmitted band of a 120 channel system, assuming a, channel bandwidth of 3000 cycles and double side-band transmission, will therefore be only 120 2 X3500 cycles or 840,000 cycles. If this band is transmitted by radio at a frequency of 4000 megacycles, with a frequency stability of :.0O5%, the total bandwidth in the receiver will be 840,000+400,000 cycles, or 1.24 megacycles, allowing 200,000 cycles on either side of the band for drift of the entire band due to carrier instability.

As this bandwidth is still more than twice that theoretically required for perfect utilization of the frequency space, additional provisions are made for further reduction of the band-width. These provisions consist of sending two channels on the frequency of each channel carrier. As outlined later in the theoretical discussion of the system, two carriers having the same frequency but separated in phase by may be individually modulated in the transmitting terminal and separately demodulated in the receiving terminal by multiplying the received signal by the amplitude of a demodulating carrier of the same phase and frequency as the carrier being demodulated. The phase-shifted demodulating carrier required is in practice provided by passing the in phase carrier through a phase network which alters its phase by 90.

A 120 channel system is therefore produced by utilizing the frequency space ordinarily required for 60 channels. Under these conditions; the frequency space required for a 120 channel system, assuming a 3000 cycle channel bandwidth and a channel separation of 500 cycles is 60 2 3500 cycles, or 420,000 cycles. If this spectrum is transmitted by radio at a frequency of 4000 megacycles, With a frequency tolerance of 1.005%, the drift allowance of 200,000 cycles on each side of the band, required on account of carrier instability, results in a total band-Width requirement of 420,000+400,000 cycles, or .82 megacycle. This requirement is to be compared with a theoretical optimum bandwidth of .36 megacycle applicable to perfect utilization of the frequency space, and with a bandwidth of 48.36 megacycles apparently theoretically obtainable with pre-selecting systems. It is also noted that present experimental systems, in which the channel separation is 40 megacycles, would require the impossibly large bandwidth of 4800 megacycles if extended to include 120 channels.

Three embodiments of the invention are disclosed herein. These embodiments differ principally in the manner in which the control carrier is generated, transmitted, and applied to the receiver to produce the required demodulating carriers.

For low frequency applications, such as carrier transmission over stable facilities permitting direct transmission of the original AM band, a common low frequency master control carrier is employed in the transmitter. The frequency of this master carrier is in general twice the audio band-width. E. g., if each transmitted channel is 3000 cycles Wide before modulation, the frequency of the master carrier is 6000 cps. The master carrier is applied 'to a non-linear element for the purpose of generating harmonics. Selected harmonics are then used as the channel carriers. For example, if it is desired to operate 4 channels at a transmission frequency of about 600,000 cycles, the 99th, 100th, 101st, and l02nd harmonics of the 6000 cycle master carrier may the transmitting terminal.

.-be used as channel carriers. One of these carriers is also used as the control, carrier and is ztra'nsmitted; the others may be suppressed in the interests of avoiding intelligible cross-modw 'lation. At the receiving terminal, the control carrier is selected by means of a very narrow fil- ..ter and applied to a series of multi-vibrators which successively reduce the frequency in steps 'to'restore the original master frequency of 6000 cps., at the same time eliminating the sidebands originally attached to the control carrier. The 6000 cycle output of the final multi-vibrator is then applied to a non-linear element and to appropriate filters adapted to select the proper harmonics for use as demodulating carriers. In the case cited, the 99th, 100th, 101st and -102nd harmonics are selected and applied to the respective demodulatorsfi This multi-vibrator system may also be employed for radio transmission in cases where it is possible at the receiving terminal to restore the multi-channel band to its original frequency detection or rectification. If excessive cross-modulation results from such detection, this procedure cannot be used and it is necessary to resort to frequency conversion by means of a heterodyne oscillator in order to reduce the band frequency to approximately its original value. The'drift of the oscillator then imparts unequal phase or frequency deviation to the control carrier with respect to the channel carriers, and unsatisfactory operation results. 7

Where frequency conversion is used to restore the multi-channel band to its original frequency, a second system for derivingthe control carrier is therefore employed. As in the first case, a master carrier having frequency equal to twice the audio band-width is employed. If, for exo ..zwhich.is p ef r b apa t of atombined mixer-oscillator. As in the; case qfthe systems previously described, the channelcarriers "At the receiving terminal, the received signal ample, the audio bandwidth selected is 4000- cycles, the master carrier has a frequency of 8000 cycles. At the transmitting terminal this master carrier modulates a control carrier at an intermediate frequency, and also harmonics of the master carrier are heterodyned with the control carrier to generate the channel carriers. The

control carrier is displaced by a substantial frewhich may be in the microwave region. At the receiving terminal, the received signal is reduced to a suitable intermediate frequency by a mixer and heterodyne oscillator. The modulated control carrier is separated from the channel carriers by means of a band pass filter sufficiently wide to accommodate the drift of the local oscillator, and is rectified to restore the original 8000 cycle master carrier. Harmonics of this master carrier are then heterodyned with the control carrier to generate the demodulating carriers, exactly as in With this system changes in the frequency or phase of the local oscillator are imparted equally to the demodulat- -ing carriers and their corresponding channel carriers, which remain in' phase at all times.

The third system of operation is applicable to direct transmission of the multi-channel band or to cases where detection can be employed in the receiving terminal to restore the multi-channel band to its original position. This system is particularly effective in conjunction with frequency modulation of a radio-frequency carrier. At the transmitting terminal, each channel carrier. is independently generated by a separate lator, which is pulled into step withlfth heterodyned with 'a local oscillator, amplifiedat an intermediate frequency and-impressedona discriminator; which restores the original multichannel band. After amplification, the ba'ridls then impressed on' the channel qemosiimterem parallel. Each "demodulator has its "own il- R carrier by injecting a small amount'of the signal voltage into the oscillator; althouglilvolta'gesfrom all .carriers enter each oscillaton; the oscillator pulls into step only with the particular 'ca'rrir to which it is most closely tunedl The out u't of the oscillator is then impressed onthe de lator to bring about product demodulatio previously described. In practice, the demoduialtor and oscillator may be contained in the'same be. 1 I '1 5 J It is an object of my invention to produce a multi-channel transmission system in'which preselection of the desired channel at the receiver is dispensed with. v

It is another object of my invention to provide means whereby, in a multi-c'hannel transmission system, twochannels maybe transmitted at each carrier frequency, through employment )of two carriers in quadrature at each frequency l A further object of my invention isto eliminate excessive separations between .channels, required in conventional preselecting radio systems on account of carrier instability, by so applyingjthe modulation to a common radio frequency'carrier that all channels drift together as the carrier varies in frequency, maintaining a constant frequency relationship between themselves,. thereby reducing the margin between the edges of the transmitted band and the cutoff frequencies of the R. F., I. F. and carrier amplifiers to that which would be required to accommodate,the'instability of the unmodulated carrier.

It is a further object of the invention to effectuate the transmission of a multi-channel spectrum superimposed on a, radio frequency; carrier without excessive interchannel cross-modulation in the receiver, through employment of a system of demodulation in which the demodulating carrier is applied to a separateterminal of the demodulator making it unnecessary to resort to non-linear demodulatorcharacteristics affecting the entire received spectrum; I i

An additional object of the invention is to provide a demodulator of the electron-coupled type in which the output current contains the necessary pure product of the channel being demodulated, and the demodulating carrier applied-to a separate terminal of the demodulator; while at the same time, products between channels are eliminated.

It is a further object of the invention "to improve the signal to noise ratio of a multi-ch'annel transmission system by reducing the band-width requirements of each channel to those actually needed for the transmission of the channel.

Another object of the inventionis to reduce the adverse effect of carrier instability and make pas?- sible the employment of less'stable and less costly est te hr ma in? t e. a e" ever amines "faruerivaeen of dnio'diilating ioar ners. Figure 4-8 "ohar'ifil' 'sy'stemnipms ine modulated control carrier.-

:mre'iienay' alloe'ations 111:1; F. stages "for" frequency conversion method applied to were:wavatransfiiissroa Figure 6-8 channel system eiiiployingi'separate fsfiehiofiihd oseillatofs. 7-'- De't'siiled seh'ei'rijtie 'oir'cui't'of synchrotimed"osnmtar amddmatcsrs "used in FlgureB.

Figure l 'demonstrates the basic theory of the t'y-peiof rfp'rodue-t 'demodulation employed in this invention. This rtheory' -:-is outlined in detailin the -teehnieal 'pap'er, Selective demodulation by 213. BxHarris, 'publlsh'ed' -in"- the J un'e 1947 issue of the Proceedings of the Institute oi -Radio "Engi'neersw Th'e followi-ng brief di'gest presents- -the salient-points of the analysis' as it is applicable *tO thiS invention.

* In :all casessa type of demodulator is employed in =-whioh the output is a linear fu'netion' of the product of the'inistantaneous: amplitude of a voltage appli'ed to on'e" terminal "of the demodulator,

and the instantaneous amplitude of-a second voltage" applied *to -another termlnal of the demodulator. lhi'srelationship may: be stated *mathematically hy the equation:-

ia teled I (1) W here z'ui is 'the lnstantaneous output :current, elus- 'the-voltage-applied to the input terminal, ea ls-a demodulating"voltage-applied to anoth'er =termlnal,= al'ld":'y is a; constant depending on the structure of the demodulator;

1f now,;-the-input-voltage -is--a modulated wave oftheiorm I -ei=Fa1-(t) cosmnt ('2) 'Whe're' FMD Y1 ls a;runeuemeememlng a moau -=latin'g voltage; and wiis -2'vr -tinres the carrier -frequen'ey; f1; and if the demo'dulatingfivoltage' is eg-1 :50am: (-3) then the output current, from Equation l will be -'2'u'-' 'iF ,-(t)i'cos witcosait yF (tycs uiil V F'-;1o. Frau) 4 f'YT'l? This'oiitiiiiteurrent isseen totcontain the'original F516), and a modulated Waite Having" a tamer. frequency offtuiiee' the I a .az e ima ma m-i amt 'w my new be" passed through a low-pass "filter,

which-"eliminates the =high frequencyrmodulated wava' leaving only the original modulating function -Faflth Complete demodulation is "thus efieet'ed.

FigurelA shows the demodulation of amodulatedoosine'carrienin' accordance with Equations 1 1104. The modulated wave, Fa1(t):cos-uw1t,:is applied 'tothe left'hand or input terminals'of demodulatorl, while at the same time, the? dem'o'dulating carrier, cos 1011?, is applied to the bottom terminals. The output of the demodulator is then in Ja'cc'or'dance with-Equation 4. The lowpass filter, 2,'e1imlnates the high frequency component, leaving the original modulating function for delivery at'theoutput of the filter.

.In a similar manner, a modulated sinecarrier may -be demodulated by multiplying-by a sine demodulating'carrier, as shown in Figure lB. In this case, an input wave,

ismultiplied in demodulator l by va demodullatting carrier,

to give, in accordance. with Equation 1; an .output current, 1 v

whioh "when "passed through *IOW passfilteri is reduced to the "original modulatingfunction H e v 7 .2 Figure 1C s'liow's -that two modulated waves having 'ca rrier's' -of the same frequency but differin'g-- in phase by may" be independentlydemodula-ted in a prod'uct demodulator I of 'th-is-ftype.

ei=Fa1 (t) cosait-i-'Fei(tllsin wit I 1168:)

where-l mit) is a econd modulating:funeti'cntv 1f tl'ie demodulating earrier alpplied to the'=bottom'it'erminals of the dmodulaton I, ismsshown in the figure,"

' I -(9) the0utput of-the demodulator}in aeoordainee with 'Eduatiori -l isr *term' is zero; This demodulator output is now impressed on "low I pass -'filter 2; l whieheliminates tha doubIe' fre'quency term's} le'a'v-ing the desired 'meduianzig funcuan,

' yam-1,

" -rn'e-snnu r manhunt dan' 'b'e'proven that if the input wave of Equation 8 is multiplied in demodulator l by a demodulating carrier,

and the resulting output current is passed through low pass filter 2, the output of the filter will be Figure 1D shows the case where this wave is demodulated by multiplying in demodulator 2 by a demodulating carrier:

' The output of the demodulator is found, by substituting in Equation 1 and performing elementary trigonometric transformations, to be:

ea=cos wit are absent. On the other hand, as the output current is a linear function of the demodulating voltage as well, it is possible to produce at will the useful product terms which result in demodulation, and only these product terms. The conventional method of detection might be characterized as a "heterogeneous method, in which all components of the input wave are multiplied together, with resulting confusion, wasteyand creation of spurious products, in order to obtain a single useful demodulation product. The demodulation method of thi invention is an orderly method, in which the same desired result is obtained directly,,with the creation of a minimum of unwantedspurious products.

The output current of Equation 13 is now passed through filter .2 which removes all terms except the first,

the modulating function attached to the cosine carrier of frequency It is noted that the 4th term is zero, and dis-,

appears.

It is also noted that, due to the linear demodulator characteristic, there are no cross-modulation terms containing the products of the amplitudes of the various components of the input wave of Equation 11. Such cross-modulation terms result when conventional non-linear demodulators such as square-law detectors or plate detectors are employed without preselection, due tothe fact that in the case of such detectors the instantaneous output current is a function of a power of the input voltage. The application of a non-filtered spectrum of the type defined by Equation 11 to the input of such a detector therefore produces a plate current which is equal in value to the value of the expansion of a polynomial, representing the amplitude of the input wave, raised to a power not equal to-l. Such an expansion contains terms representing the product of each component of the input wave by every other component of the input wave. When the input wave contains the carriers of the various channels, some of these terms are the useful demodulation products, but others represent cross-modulation between channels. Where nonlinear detectors are employed, it is therefore necessary to eliminate the unwanted components by prefiltering before the demodulation process takes place, in order to avoid cross-modulation.

Conversely, it is possible to dispense with preliminary filtering only if a type of demodulator be employed in which the output'current is substantially a linear functionof the input voltage, as in this invention. strated by Equation 13, cross-modulation terms In this case, as demon 1 2+ 1)t+v S111 eh) demodulator l by a demodulating carrier, ed=sin wit; the output of filter 2 will be 'Y bl( I A'demodulating carrier ee=cos wzt gives i and if .ea is sin wzt, V 'Y MU) willbe obtained at the output terminals of the filter. Thus, any of the four modulated waves may be demodulated at will by choosing-a de'- modulating carrier having a frequency and phase angle identical with that of the carrier being demodulated.

Since um and wz represent any two frequencies,"- it is evident that this principlemay be extended to cover the demodulation of a series of modulated-waves having diiferent carrier frequencies,

a cosine carrier and a sine carrier being employed at each of the chosen frequencies in the series. It is of course necessary: that low pass filter 2 have a characteristic such that it will cut-oil. the

unwanted demodulation products and pass' the desired modulating function; and that the carrier frequencies be so spaced as to render this and that the carriers shall'be spaced, twice this distance apart in frequency, assumingthat all modulating functions have the same upper frequency limits. The applicationof a numerical exampleto Equation 13 will make-thisclear. If we assume that-the modulating functions .Feitt), Fb1 (t Fa2(t) and Fbzit) :are audio frequency bands having upper limits at 3000 cps. we see that in accordance with the above requirement, the carriers must be. spaced 6000 cycles apart. IL'then will. be 16,000 cps. Terms 2 and 3 will then represent modulated waves having carrier frequencies at 32,000 cps. and lower and upper sidebands terminating at 29,000 and 35,000 cps. respectively. Term 4 is zero, and vanishes. Terms 5 .andfi havecarriers at 16,000-10,000 or, 6000 cps. with lower and upper sidebands terminating at 3000 cps. and 9000 cps. respectively. The carrier frequency of terms 6 and 7 is 36,000 cps. and their sidebands extend from 33,000 to 39,000 cps. Under. theseconditions all terms except the first, Fair), which has a top frequency of .3000 cps. arejeli'minated by the 3000 cycle cutoff filter. It is to'bei'observed, however; that if the carriers had been spaced less than 6000 cycles apart, or if the cutoff frequency of the filter had been higher than 3000 cycles; terms 5 and 8 would have contained components which would. have passed through the filter. I

In practice it is,lof course, impossible to construct a filter which will cut off so sharply as to cause infinite attenuation above the cutofi frequency, and none below it. It is, therefore, necessaryin the -practical design of the circuit, to space th'e ca'rriers a little farther apart than twice the highest frequency in the modulating functions.

In some applications it is possible to dispense with filter 2 entirely,-as for example where the load circuit to which the output of demodulator l is delivered is not responsive to the extraneous terms of higher frequency, and their suppression is therefore not required.

It is to be noted that the action of filter 2 is equivalent to "averaging or integrating the amplitude of the wave delivered to it by demodulat'or I. The combined action of demodulator l andrfilter l may therefore be said to result in integratin'g'the product of the amplitude of the demodulating: carrier and the amplitude of the input'wave.

It may easily be shown, in fact, that'the most general mathematical expression for the combined action of the product demodulator and filter is an equation stated in the integral form. For the average output current obtained at the output of the filter. isrseen' to be:

where Q is the quantity of electricity flowing throughout an averaging interval of length T. In turn the value ofQ may be found from the relatiol'iship roedQ=io Q=L io (13 so that if in be of the form of Equationl, we have, bysubstitution in Equation 13A:

If now the input wave be of the form of Equation 11, generalized fora series of terms,

is afundamental carrier frequency, of which all other'carriers in the spectrum are harmonics, we

then have, by substitution in Equation 13C, the relation:

T i IU,=%J; [ZF Q) cos mot l-21 ,10 sin nwtle di f (13a) The demodulating wave may beexpressed in most general form as:

ee=cos nwt (13F) or ee=sin-mvt (13G) depending upon whether-demodulation of the cosine or of the sine channel is being effected. It is also seen that if the period of integration is taken to be the period of the fundamental carrier,

(13E) to be:

2w w E I ZF (t) cos nwH- ZF,,,, (i) sin n'wt] cos moi dt Jan-o T (13H) and respectively.

These equations express the overall operation of the combined system, and the averaging action of the filter in the most precise possible mathematical form. It is also seen that the function of integration might be carried :out by means other than a filter, as for example, by anintegrating circuit adapted to integrate the demodulator output wave over one exact cycle of the fundamental carrier frequency.

Figure 2 shows a practical product demodulator of the type described in general in Figure 1, employing a 6L? pentagrid mixer tube. As indicated in Figure 2B, the center portion of the characteristic of the tube relating the oscillator grid .voltageee, to the signal grid-plate transconductance gm, is substantially a straight line, defined bythe equation,

Where gm is thesignal grid to plate transconductance, cu is'the voltage value at which an extension ofthe straight portion of the characteristic intersects the horizontal axis, es is the instanta- 13' neous valued the voltage applied to the oscillator grid, and v is a constant representing the slope of the curve.

'For values of load impedance which are small with respect to the plate impedance, the plate current, in is given by:

I to (gm1i+gmdd) where e1 is the voltage applied to the signal grid, and gmd is the oscillator grid-plate transconductance.

Substituting (14) in (15) we obtain,

I 2'0='y (err-6o) 6i+gmd6d (16) =76d8i-7606t-I-Qmd6d Or, neglecting the high frequency 2nd and 3rd terms, which will not pass through the output filter, 7

This equation is seen to be identical with Equation 1, which expresses the relationship required in order to bring about product demodulation.

If an incoming modulated wave having an instantaneous amplitude, ex is therefore impressed on the signal grid of the tube, and a demodulating carrier is impressed on the oscillator grid, the relationships described in Figure 1 and defined by Equations 1 to 13 apply.

In practice, as shown in Figure 2A, the incoming signal e1, after passing through transformer 3, the secondary winding of which is tuned by condenser- 4 to the'frequency of the carrier being demodulated, is impressed on signal grid 5. The de modulating carrier, ea, is delivered to oscillator grid 1 by transformer H, the secondary winding of which is tuned to the frequency of the demodulating carrier by condenser l2. Grids 5 and 8 perform the usual and well-known functions of a screen grid, being maintained at the proper potential by voltages derived from the plate supply, B+, through resistor l3. Grid 9 is a suppressor grid provided for the purpose of suppressing secondary emission. Cathode I4 is maintained at the proper bias by bias resistor i5 which is shunted by condenser IS in order to bypass alternating current. The output of the tube, in, containing the wanted demodulation product, and the higher frequency extraneous components, is delivered by transformer I! to the low pass output filter, which comprises inductances i8, i9, and 2! and condensers 22, 23 and 24. Operating in the usual and well-known manner of low-pass filters, this filter removes the unwanted product of demodulation, and delivers the particular modulating function, F(t) which is wanted, at its output terminals.

Figure 3 is a block diagram showing a complete 8 channel system of the first type mentioned previously, in which the demodulating carriers are derived in the receiving terminal by means of multi-vibrators. The demodulators in this system are identical with Figure 2A; as the operation of all other components is well known, a detailed circuit drawing is not provided.

The eight channels are identified in Figure 3 as channels IA, 5B, 2A, 23, 3A, 33, 4A and 4B, and it is assumed that the modulating functions applied to the inputs of these channels at the transmitting terminal are Fad), Fb1(t), Fazhf), Fwd), Fmt), Fred), Fa4(t) and-F114), respectively. Multivibrator 25 constitutes a common source for all carriers used in the system. This multivibrator, operating at a fundamental frequency of 6000 cpsi, twice the band width of the modulating audio frequency-signal, generates a square topped wave which is rich in harmonics."

Its output is impressed in parallel on carrier amplifiers 26, 21, 28 and 29, which are tuned sharply to the particular harmonics which it is desiredto use as channel carriers. Considering the operation of channels IA and B, the carrier balanced modulator, arranged to generate adouble side-band, carrier suppressed wave. The

outputof this modulator will therefore be ofthetype defined by Equation 2;

. a1=Fal(t) cos wit (18) in which I Phase network 31 is designed to transmits, wave delayed with respect to the output of phase network 30; or in other words, a sine wave.

This wave is used as the channel carrier of 'modu' later 39, which is also a balanced modulatorgenerating a double side band, carrier suppressed wave. The output of modulator 39 is-therefore of the type defined by Equation 5;

eb1=Fb1(t) sin wit in which The structure of the channel 2 transmittingequipment, including carrier amplifier 21, phase; networks 32 and 33', and modulator 40 and 4|.

is identical with that described above for channel I, except that the carrier frequency selected by carrier amplifier 21 is the th harmonic,.

600 kc. and modulator 40 is slightly unbalanced for the purpose of transmitting a small amount of the 600 kc. cosine carrier to the line. This carrier functions in the receiving terminal as a con-.

trol carrier, from which the demodulating carriers are derived. The outputs of modulators 40 and 4| are therefore, respectively:

caz=Faz(t) cos 'wct-i-E cos wt (20) and 8172=Fb2 t sin wet- (21) Where wg -fz-GOOXIW and E is the amplitude of the unmodulated carrier transmitted.

In a similar manner, carrier amplifier 28 in channel 3 selects the 101st harmonic of the multivibrator frequency, 606 kc., and delivers it through cosine phase network 34 and sine phase network 35 to balanced modulators 42 and 43 respectively; and carrier amplifier 23 in channel 4 selects the 102nd harmonic, 612kc., for delivery to balanced modulators 44 and 45 through cosine phase network 36 and sine phase network 3'! respectively. The outputs of modulators 42, 43, 44' and 45 are therefore, respectively,

a3=Fa3(t) cos wat (22) eb3=Fba(t) sin war (23) 6a4=Fa4(t) cos 10d (24) and;

6b4=Fb4(t) sin and (25) The" eight modulated waves arev now impressed in parallel on hand amplifier 46-, which amplifies thenrcollaterally, and delivers them to line 41.

At the receiving terminal, receiving band amplifierlaagain amplifies the entire band of frequencies, arid-impressesit inparallel oncarrier l minal, and perform the function of selecting; the 99th harmonic, 59 iv kc., the 106th harmonic. 600: kc., the 101st harmonic, 606 kc., and the. 102nd harmonic, 612 kc., for use.- as the demodulating carriers of channels I, 2, 3 and 4, respectively.

Considering the operation of channel I only, the 594 kc. wave delivered by carrier amplifier 55 is transmitted as a cosine wave cos 10115 through phase network 59 tov demodulator 61. The output. of demodulator. 67 is the product of the input wave c1 defined by Equation 26, and the demodulating carrier Ed CDS wit. The operation of Equation 13 is therefore performed by demodulator 61, which delivers to the output terminals of channel IA the original modulating function where K is a constant depending on the lossin 2O Fsiit) which was associated with that channel the line and the gain in band amplifiers 46 and 48, and the expression in brackets is the sum of the wave emitted by the individual transmittingmodulators, as defined by Equations 18 to 25, inclusive.

Ca-rrier amplifier 49' selectsthe final term in Equation. 26,. E cos. wit, amplifies it and impresses it on'multi-vibrator 50. It is to be noted that although the 3rd and ith-terms in Equation 26-also have rcarrier frequencies these terms actually represent sideband extend-'- ing to both sides of the carrier with no carrier present. Since carrier amplifier 49 is very sharply tuned it passes very little of these sidebands; and the voltage impressed on multi-vibrator 50 isfundamently that of the 600 kc. control carrier.

Multi-vibrator 50 operates at a fundamental frequency of .60 kc. In the well-known-manner of multi vibrators, it therefore emits a 60 kc. fiat topped wave, which is automatically synchronized=with the 600 kc. control carrier. The output of'multi-vibrator 50 is impressed on 60 kc. amplifier 5|, which amplifies the 60 kc. wave, removes the harmonics, further attenuates that small amount of side-band component which passed through amplifier 48 and multi-vibrator 56;.and impresses the 60 kc. wave on multi-vibrator 52.

Multi-vibrator 52 operates at a fundamental frequency of 6 kc. in synchronism with the 60 kc. wave derived from multi-vibrator 50, and therefore also in synchronism withv the 600 kc. control carrier. The 600 kc. control carrier is in turn a harmonic of the wave emitted by the master multi-vibrator in the transmitting terminal. The outputof multi-vibrator 52 is therefore identical-in-frequency with that of multi-vibrator 25 andit diifers in phase from that of multi-vibrator'25' only by the amount of phase shifts caused by the line and. by the band amplifiers, which affect all channel carriers equally.

The 6 kc. wave emitted by multi-vibrator 52 is amplified by 6 kc; amplifier 53, which further attenuates any residual harmonic or side-band components; delivering a practically pure 6 kc. wave to multi-vibrator 54. Multi-vibrator 54 also operates at 6 kc., and emits a fiat-topped wave, richiin harmonics, which is impressed in parallel on carrier amplifiers 55, 56, 51, and 58. These carrier amplifiers are identical with carrier amplifies 26, 21, 28, and 29 in the transmitting terat the transmitting terminal.

The 594 kc. output of carrier amplifier 55 is also delivered as a sine wave, sin wit; through phase network 66 to demodulator 68. Demodulater 68 multiplies the input wave e1 as defined by Equation 26, by the demodulating carrier erz=sin wit to produce at the output terminals of channel lB Fbl(t) the modulating function originally associated with that channel at the transmitting terminal.

The th harmonic of multi-vibrator 54, having a frequency of 600 kc., is, in a similar manner. selected by carrier amplifier 56 andappliedas a cosine function to demodulator 69, through phase network 6| and as a sine function to demodulator 10 through phase network 62, to pro duce, at the output of channels 2A and 2B, the

modulating functions Pant) and Fb2(t) which were originally associated with those channels at the transmitting terminals. Carrier amplifier 51 selects the 101st harmonic, 606 kc. and applies'it. through cosine phase network 63 to demodulator H and through sine phase network 64to demodulator 72, to produce at the output terminals of channels 3A and 33, respectively. the correct modulating functions Pant) and Fb3(t). In the same manner, carrier amplifier 58, phase networks 65 and 66, and demodulators 13 and M produce at the output of channels 4A and 4B; the original functions Feat) and F114).

Thus, the entire complex wave is demodulated,

the respective modulatingfunctions being deliv-- ered at the proper output terminals of the receiving terminals.

as previously explained, the system of Figure 3 1s successful when it is possible to maintain a high degree of stability in the'frequencies of the channel and control carriers. This condition will in general obtain when wire transmission at. the originalmoclulated frequency can be employed, or when theentire low frequency modulated band can be used to modulated a radio frequency carrier which is in turn'rectified to restore the low frequency band in the receiving terminal. If considerations relating to cross-modulation make this impossible, a second method of operation, shown in Figure 4, isutilized. In the system of Figure 4, radio transmission is employed, but cross-modulation is eliminated by. avoiding. the use of rectifiers or detectors in the channel equipment of the receiving terminal. The'multi-channel band is restored to approximately its original position in the spectrum bymeans of a heterodyneoscillator, and means are incorporated to permit this oscillator to drift infrequency-with- 17 out disturbing the selection of the demodulating carriers.

In Figure 4, the fundamental frequency for all carriers is derived from oscillator 80which operates at a frequency of 8000 cps. equal to twice the audio band-width, taken in the case of this example to be 4000 cps., and from oscillator 8!, which emits a control carrierat a nominal intermediate frequency, in this case taken to be 29.736 megacycles per second. Harmonics of oscillator 80 are heterodyned with the output of oscillator 8| to provide the channel carriers. In order to develop these harmonics, oscillator 80 triggers multi-vibrator 82, which also operates at a fundamental frequency of 8000 cps., and has an output rich in harmonics. Carrier amplifiers 83, 84, .85 and 86 select the proper harmonics for their respective channels, and impess them on mixers 81, 88, 89 and 90, where they are heterodyned with the control carrier, 29.736 me. to produce sum frequencies which are used as the channel carriers.

This process can be made clear by considering the operation of channel I. In this channel, carrier amplifier 83 selects the 64th harmonic of multi-vibrator 82, 512 kc., amplifies it, and delivers it to one input terminal of mixer 81, on the other input terminal of which is impressed the control carrier, 29.736 me. In the well known manner of mixers, mixer 81 produces the sum and. difference frequencies of these two input waves. The sum frequency, 30.248 me. is selected by the output circuit of the mixer, and is delivered through phase network 9| as a cosine wave to modulator 99, and through phase network 92 as a sine wave to modulator I00. of channels IA and IB are therefore The outputs 3al=Fa1(t) cos wt (27) and 8b1:Fbi(t) sin wit (-28) where selects the 66th harmonic, 528 kc., and delivers it to mixer 89, which heterodynes it with the control carrier to produce the sum frequency, 30.264 me. which is used as the channel carrier of channel 3, applied to modulator I03 through phase network 95 as a cosine wave and to modulator I04 through phase network 96 as a sine wave. The 67th harmonic, 536 kc., is selected by carrier amplifier 86, and added to the control carrier by mixer 90 to produce the channel 4 carrier, 30.272 mc., which is applied to modulator I05 through phase network 91 as a cosine wave, and to modulator I06 through phase network 98 as a sine wave. The outputs of modulators IOI, I02, I03,

I04, I05 and I08 are therefore, respectively,

a2:Fa2(t) cos wt (29) eb2:Fbz(t) sin wzt (30) 6a3:Fa3(t) cos wt (31) eb3:Fb3(t) sin wst (32) ea4:Fa4(t) COS w it (33) eb4:Fb4(t) sin um? (34) where designed to generate a double-sideband modulated wave with the carrier transmitted, so that its output, in accordance withthe well known theory of modulators of this typeis:

ec:E(1+wm cos wmt) cos wkt where y =8000 cps., =29.736 mo.

and m is the modulation index, which, in this case, is made very small, for reasons to be explained later; in other words, the output of modulator I01 is almost pure carrier, with only a small amount of modulation. v

The outputs of modulators 99, 'I00,IOI, I02, I03, I04, I05, I06 and I0! are'now impressed in parallel on radio transmitter I08, to form a multi-channel band which modulates the transmitter. The output of transmitter I08, which may be in the micro-wave'region, is transmitted through antenna I09 to the receiving terminal, where it is received by antenna I I0 and impressed on radio frequency amplifier III. Amplifier III amplifies the signal and delivers it to mixer I I3, which in conjunction with local oscillator I-I2, performs a frequency conversion in the conventional manner and reduces the frequency of the signal to the 30 megacycle range. The multichannel band used to modulate the radio transmitter is thus reestablished in approximately its original position in the frequency spectrum. In the following discussion it is assumed that the frequency of local oscillator I I2 is exactly correct to reestablish the original frequencies, and that the output of mixer II3 therefore contains 4 channel carrier frequencies, at 30.248 mc., 30.256 mc., 30.264 mc., and 30.272 mc., and'the control carrier, at 29.736 mc. p I Carrier amplifier II4 excludes the channel carriers but selects the control carrier 29.736 mc. and impresses it on detector II5, which rectifies it to restore the original 8000 cycle modulation. The output of detector H5 is thus an 8000 cycle wave identical in frequency and phase with the wave produced at the transmitting terminal by oscillator 80, from which all channel carriers were derived. This wave triggers multi-vibrator II6, which produces a square-topped 8000 cycle wave, rich in harmonics, and impresses it in parallel on carrier amplifiers H8, H9, I20, and I2I. These amplifiers select, respectively, the 64th harmonic, 512 kc., the 65th harmonic, 520 kc., the 66th harmonic, 528. kc., and the 67th harmonic, 536 kc., and impress these harmonics on one input terminal of each of mixers I22, I23, I24 and I25, respectively. The other input terminal of each of these mixers is supplied with the control carrier, 29.736 mc. The sum frequency outputs of mixers I22, I23, I24 and I25 are therefore waves having frequencies of 30.248 mc., 30.256 mc., 30,264 mc., and 30,272 mc., respectively. These sum frequencies are transmitted as cosine demodulating carriers through phase networks I20, I28, I30 and I32 to demodulators I34, I36, I38 and I40 respectively; and as sine demodulating carriers through 19 phase networks I27, I29, HI and I33 to demodulators I35, I 31, I39 and MI respectively.

I. ;F. amplifier II'I excludes the control carrier at2-9.736 mc., but selects the channel carriers at 30.248 mc., 30.256 mc., 30.264 mo. and 30.272 mc., and delivers them in parallel to demodulators I34, I35, I36, I31, I38, I39, I40 and MI.

Since the channel carriers and the demodulating carriers were all derived from a common source, oscillatorin'the transmitting terminal, each demodulator has. a demodulating carrier which is identical in phase and frequency with one of the channel carriers. It therefore demodulates that particular carrier, and delivers the original modulating function at its output terminals. The demodulating carriers have been so allocated to the demodulators that the output of channel IA is Fa1(t), of channel IB, Find) of channel 2A FB.2(t), of channel 2B, F'b2(t), of channel 3A, Fa3(t), of channel 33, Feed), .of channel 4A, Flint), and of channel 4B, F h4(t), in each case the modulation which was impressed on the corresponding channel-at the transmitting terminal. Complete and selective demodulation is thus efiected.

The modulation index of the control .carrier defined byEquation 35 is kept very low because if any appreciable amount of modulation were present in that portion of theoutput of carrier amplifier H4 which is delivered to mixers,fI22, I23, I24, and I25, the output of these mixers would not be the pure carriers required for demodulating purposes, but would contain frequencies representing the sum of the respective harmonics of multi vibrator H6 and the side bands associated with the control carrier. These components would lie at points in the spectrum occupied by channel carriers, and, in any given demodulator, would produce demodulation of channel carriers properly associated with adjacent demodulators. As the modulation index is actually very low, however, the amplitude of the side-bandsassociated with the control carrier is also very low, with respectto the control carrier, and components in thedemodulating carriers resulting from mixing these side-bands with harmonics of multi-vibrator I I6 are negligibly small. It is possible to employ a low modulating level of this nature in the control carrier without introducing excessive noise at the input to multivibrator H6, because the output pass-band of detector H is extremely narrow, and admits noise lying only a few cycles to either side of the 8000 cycle output.

as is actually the case, the frequencies of the channel and control carriers at the output of mixer I I3 do not coincide with their counterparts in the transmitting terminal but differ widely from them due to drift in the frequency of the transmitting or receiving oscillators, the control carrier still maintains a fixed relationship to the channel carriers, and produces demodulating carriers identical in phase and frequency with the channel carriers. Drift in the transmitting and receiving oscillators is therefore of no consequence in maintaining satisfactory operation of the system, providing that it is not so wide as to cause the channel carriers or the control carrier to drift outside the pass-band of I. F. amplifier H! and carrier amplifier H4, respectively. Figure 5 shows the frequency allocations which have been selected for the carriers and their associated amplifiers. The pass-band of I. F. amplifier II'I extends from 30,045 me. to 30.475 me. The channel carriers with their side bands occupy 20 a space only 30 kc. wide, from 30.245 me. to 30.275 mc. in the center of this pass-band. Provision is therefore made for the'message carriers to drift 200 kc. to either side of their assigned position without leaving the pass-band of the I. F. amplifier. If the transmitter operates at a micro-wave frequency, as for example-i000 mc., this amount of drift 1-200 kc., represents a frequency stability of :.005%, a tolerance which can be achieved in practice.

Similarly, carrier amplifier I M has a pass-band extending from 29.625 mc. to 29.945 me. The modulated control carrier occupies a space 16 kc. wide, from 29.728 mc. to 29.744 mc. .in the center of this band, leaving a margin of about 200 kc. on either side for frequency drift.

It is seen from Figure 5 that any phase or frequency instability in the transmitting or receiving equipment will be imparted equally to the channel carriers and control carriers, which will always maintain a fixed frequency separation (amounting, for example to 30248-20736 or .512 mo. in the case of channel I). In generating the demodulating carriers, the frequency added to the control carrier is exactly equal to this separation, and as the added frequency is derived from oscillator in the transmitter, the value added is identical both in the transmitter and in the receiver. It is thus seen that frequency instability has no eifect on the operation of the system.

Figure 6 discloses a simpler system in which each channel carrier at the transmitting terminal and each demodulating carrier at the receiving terminal is individually derived from a separate oscillator for each channel. This system can be employed where considerations relating to cross-modulation are such as to permit the use of a detector or discriminator at the receiving terminal in place of a heterodyne oscillator, for the purpose of restoring the multi-channel band to its original position in the frequency spectrum; or where transmission is by wire line. At the transmitting terminal, channel modulators I50, I52, I54 and I56 receive cosine channel carriersfrom oscillators I58, I60, I62 and I64 respectively, and these same oscillators furnish sine channel carriers to modulators I5I, I53, I55 and I57 respectively, through phase networks I59, I6I, I63 and I65 respectively. Oscillators I50. I52, I 54 and I56 are separated in frequency by twice the band width of the individual channel modulating signals, plus an allowance for drift of each oscillator. In this case, this drift allowance is made considerably larger than in the case of Figures 3 and 4, since the oscillators operate independently; and the operating frequencies of oscillators I58, I60, I62 and I64 are accordingly taken to be 2015 kc., 2005 kc., 1995 kc. and 1985 kc. respectively. The multi-channel band is placed in the 2 me. region rather than in the 600 kc. region as in Figure 3, in order to facilitate the design of the relatively wide-band amplifiers required, and to improve the deviation where frequency modulation of the radio transmitter is employed.

Modulators I5I, I53, I55 and I5! are of the double side-band carrier suppressed type employed in Figures 3 and 4, but modulators I50, I52, I54 and I 56. are arranged to transmit the carrier as well as both side-bands.

The outputs of modulators I50, I5I, I52, I53, I54, I55, I56 and I5! are impressed in parallel on radio transmitter see, which may operate in the micro-wave region. The modulation employed in transmitter I66 is preferably, but not necessarily, frequency modulation effected by the means described in patent application, Serial Number 173,693, filed Septembed 12, 1947, by D. B. Harris. In any case, transmitter I66 impresses a high frequency R. E. carrier, modulated by the multi-channel band created by modulators I50, II, I52, I53, I54, I55, I56 and I51, 'on antenna I89,.whi-ch emits a corresponding radio wave. At

the receiving terminal, this wave is picked up by antenna I90, selected and amplified by R. F. amplifier I61, heterodyned to a lower intermediate frequency (as 30 me.) by the combined action of oscillator I69 and mixer I68, amplified by I. F. amplifier I10, and impressed on discriminator I1I (which may be a detector if amplitude modulation is employed). Discriminator I1I demodulates the wave, restoring the original multi-channel band at its original position in the frequency spectrum. The multi-channel band is amplified by band amplifier I12, and impressed in parallel on the input terminals of demodulators I81, I82,

I83, I94, I85, I86, I81 and I88.

A portion of the multi-channel band voltage is also impressed in parallel on oscillators I13, I15, I11 and I19, which are tuned approximately to 2015 kc., 2005 kc., 1995 kc.,' and 1985 kc. respectively, the'frequencies in the multi-channel band. Each oscillator is accordingly subject to the influence of the injected voltage of a cosine wave having a frequency close to its own natural frequency of oscillation, and in accordance with the well-known principles of operation of oscillators triggered by injected voltages, pulls into step with that voltage. Oscillators I13, I15, I11 and I19 therefore emit demodulating carriers consisting of cosine Waves having frequencies of 2015 kc., 2005 kc., 1995 kc., and 1985 kc., respectively, identical in phase and frequency with the cosine channel carriers at those frequencies. These demodulatingcarriers are impressed on demodulators IBI, I83, I85 and I81, which carry out the process of product demodulation in the manner previouslydescribed, delivering at their respective outputs the modulating functions originally associated, at the transmitting terminal, with their respective channels, Fal(t), Ferd), Fa3(t), and

FII-i(t) I I A portion of the outputs of oscillators I13, I15, I11, and I19 is also delivered through phase networks I14, I16, I18, and I80 respectively, in the form of sine demodulating carriers, to demodulators I82, I84, I86 and I88, respectively, which demodulate the sine components of the multi-channel band in accordance with the principles of product demodulation, and deliver at their respective outputs the modulating functions originally associated at the transmitting terminal with their respective channels Fbdt), Fsz(t), F'b3(t) and F1146) Complete and selective demodulation of th input Wave is therefore effected.

In the case of Figure 6, it is seen that frequency drift in the radio frequency section has no effect on the frequency of the components of the multichannel bandin the receiver since a detector or discriminator, rather than a heterodyne oscillator is used to restore the multi-channel band to its original position in the spectrum. The only requirement relating to frequency stability is therefore that the receiving oscillator-s shall be able to follow the frequency variation of, and remain in step with the transmitting oscillators. This requirement can be met by reasonably careful design of the transmitting and receiving oscillators.

Figure 7 is a detailed circuit drawing of the oscillator-demodulator assembly used at the receiv-' ing terminal for demodulating each pair of channels as indicated in block form for channel I,;for example, by the combination of element-s I13, I14, NH, and I82, of Figure 6; In Figure 7, a common vacuum tube, indicated as tube A, contains the demodulator for the cosine channel and the oscillator, avoiding the use of an external oscillator. Tube A is a pentagrid converter such as the 6A8, containing cathode 236, grids 231, 238, 239, 240 and MI, and plate 242, all elements being connected in the circuit in a conventional manner, which need not be described in detail. The supply for oscillator grids 231 and 238 is derived from the tuned oscillating circuit consisting of transformer windings 201 and 206, andcondenser 2I0. The ground return from Winding 201 passes through synchronizing potentiometer 25I on the input terminals of which is impressed a portion of the input signal wave, through condenser 250; injected voltage components of the signal wave, appearing across potentiometer 25I are therefore delivered to grid 231, causing grids 231 and 238 to oscillate at the frequency of and in phase with that particular cosine channel carrier to which they are most closely tuned by the setting of condenser ZIII (sine carriers are not transmitted).

The product demodulation of that particular cosine carrier is therefore performed by tube A in the maner previously described, the resulting modulating function being delivered to the output terminals of the channel through output transformer 2 l6 and a low pass filter comprising inductive reactances 2I8, 219, 220 and HI, and condensers 222, 223 and 224.

The input wave enters the circuit through a tuned input circuit consisting of transformer 200 and condenser 20 I, which also delivers the input Wave, in parallel, to the synchronizing potentiometer 25I and signal grid 244 of tube B. Tube B is a pentagrid mixer tube such as the 6L7 containing cathode 243, grids 244, 245, 246, 241 and I 246, and plate 249, all elements being connected in the circuit in a conventional manner which need not be described in detail. The supplyfor oscillator grid 246 is derived from a tertiary windm 209, of the oscillating circuit associated with tube A, in parallel with tuning condenser 2| I, and through a phase network consisting of resistors 2I2 and 2I3, and condensers 2M and 2I5, which alters the phase of the Wave by If We consider the demodulating carrier impressed on grids 231 and 236 of tube A to be a cosine wave, the dem-odulating carrier impressed on'grid 246 of tube B is accordingly asine wave. Tube B therefore demodulates the sine channel carrier on the same frequency as the cosine channel carrier demodulated by tube A, and delivers the resulting modulating function at the output terminals of the channel through output transformer 2I1 and a low pass filter comprising inductive 'reactances 225, 226, 221 and 226, and condensers 229, 230 and 23I.

Resistors 202 and 204 and condensers 203 and 265 maintain the grid bias of tubes A and B respectively, at the center of the linear portions of their characteristic curves, as demonstrated in Figure 2B, and their screen grids are supplied by resistors 234 and 235, respectively. B voltage for the oscillator section of tube A is pro vided by resistor 232. Condenser 233 provides a bypass for high frequency around this resiston In practice the setting of synchronizing potentiometer 25! is kept as low as possible, consistent with adequate synchronization, in order to reduce to the greatest possible extent the transmission through tube A of extraneous sideband and carrier components, which might cause cross-modulation or distortion in both tubes. Experience shows that there is no diiiiculty in finding a setting of potentiometer 25! such that extraneous components are suppressed below audibility, while adequate synchronization is still obtained.

vIt is emphasized that where, in the foregoing specification, mention has been made of specific frequencies, or of specific expedients implementing the general principles involved, such mention has been made for illustrative purposes only, and is not to be construed as limting the scope of the invention to the expedients and frequencies mentioned. For .example, audio-frequency, carrier frequency, video-frequency, or radio frequency waves, or light waves may be used in the transmission portions of the system. Similarly, the modulation applied to the individual channels need not necessarily'be audio frequency, but may be radio-frequency, video-frequency, or may even be on-ofi D. C. where it is desired to use one or more'channels for the control of servo mechanisms, or for telephone supervision applications. In general, carriers may be either transmitted or suppressed, taking into consideration the fact that the transmission of carriers has a tendency to increase the intelligible cross-modulation. Systems of modulation used in the transmission section may be AM, FM, PM, PAM, PPM, PCM, etc. depending upon the needs of the particular application. Demodulating elements are not necessarily vacuum tubes, as illustrated in the figures, but may be transducers of any type having characteristics conforming with the requirements of the theoretical discussion of the principles involved; in other words, any transducers providing output responses linearly proportional to the product of two input signals. Inparticular, crystals or copper-oxide rectifiers having bilateral conductivity characteristics can in many cases be used in place of the vacuum tubes shown.

Although the 3 systems disclosed in detail provide cosine and sine'channels "on each carrier frequency, it is obvious that any combination of carriers may be used, and, if desired, the cosine channels only, or the sine channels only, may be employed in a mum-channel system, or certain channel frequencies may employ two carriers and others only one. The number of carriers used isalso not pertinent to the scope of the invention; for example, a complete dual-channel system may comprise a cosine carrier and a sine carrier on the same frequency; and a single channel system may have either a cosine or a sine carrier on a single frequency, demodulated by the product demodulation method.

What is claimed is:

1. In a multi-channel communication system wherein the modulated carrier wave of each channel is demodulated at the receiving terminal by multiplying the instantaneous amplitude of the received complex wave by the instantaneous amplitude of a demodulating carrier wave having the same phase and frequency as the particular carrier wave being demodulated, a system for producing said demodulating carrier waves comprising at the transmitting terminal a common multivibrator adapted to generate a low frequency fundamental wave and harmonics thereof, and filters for selecting one of said harmonics for use as the carrier wave of each channel, and at said receiving terminal, a series of frequency dividers and a filter adapted to select one of said carrier 24 waves, and to apply said carrier wave to said series of frequency dividers to control the generation of .a wave identical with said low frequency fundamental wave and harmonics, filters for selecting for use as the demodulating carrier of each channel the particular one of said harmonics which is identical with the harmonic selected at the transmitting terminal as the carrier wave of that particular channel, and a series of mixers receiving the incoming waves to mix them respectively with one of the outputs of said filters, and a biasing network connected to each of said mixer tubes for maintaining operation .of the tubes in that region of their characteristics where the signal grid-plate transductance is substantially constant with change in signal grid voltage.

2; In a multi-channel communication system wherein themodulated carrier wave of each channel is demodulated at the receiving terminal by multiplying the instantaneous amplitude of the received complex wave by the instantaneous amplitude of a demodulating carrier wave having the same phase and frequency as the particular carrier wave being demodulated, a system for producing said demodulating carrier waves comprising, at the transmitting terminal, a common multivibrator adapted to generate a low frequency fundamental wave and harmonics thereof and a common oscillator adapted to generate a wave of intermediate frequency, a filter and mixer for selecting-one of said harmonics and mixing said harmonic with said intermediate frequency wave to produce a heterodyne frequency wave used as the carrier of each channel, means for modulating Said intermediate frequency wave bysaid lowfrequency wave, and at said receiving terminal a detector for .demodulating said intermediate frequency wave to restore said .low frequency fundamental wave, a multivibrator triggered by said low frequency fundamental wave to generate harmonies of said low frequency fundamental wave, and means for mixingse'lectedharmonics with said intermediate frequency wave to produce a heterodyne frequency wave for use as the demodulating carrier of each channeL'identical in phase and frequency with the carrier of said channel, and said means for mixing comprising mixer tubes which are biased so as to maintain operation in that region of their characteristic where the signal grid-plate transductanceis substantially constant with change in signal grid voltage.

3. A multi-channel communication system comprising a transmitting terminal including oscillators and modulators associated with each channel adapted to produce for each channel a modulated carrier wave having a frequency differingfrom the frequency of the carrier wave of each other channel, and a receiving terminal having demodulators associated with each chan nel adapted to demodulate, separately and individually, each of said modulated carriers solely, by multiplying linearly the instantaneous amplitude of the complex received wave by the instantaneous amplitude of a wave having the same frequency and phase as the particular carrier being demodulated, and said demodulators comprising mixer tubes and biasing networks connected together such that the biasing networks cause operation of the mixer tubes in that region of their characteristic where the signal grid-plate transductance is substantially constant with change in signal grid voltage.

4. In a multi-channel transmission system, where'each channel of the system comprises an amplitude modulated carrier wave, displaced in other channels, a demodulating system for each channel comprising an oscillator providing a demodulating carrier Wave identical in phase and frequency with the carrier wave of said channel, a low pass filter, and a demodulator adapted to deliver to said filter a wave having an instantaneous amplitude equal to the linear product of the instantaneous amplitude of said demodulating carrier and the sum of the instantaneous amplitudes of the carrier waves of all channels, and said demodulator comprising a mixer tube and a biasing networi: connected together such that the biasing network causes operation of the mixer tube in that region of its characteristic Where the signal grid-plate transductance is substantially constant with change in signal grid voltage, and substantially proportional to the oscillator grid voltage.

5. In a multi-channel transmission system wherein each channel in a first group of channels comprises an amplitude modulated carrier wave displaced in frequency with respect to the carrier waves of other channels, and each channel in a second group of channels comprises an amplitude modulated carrier wave identical in frequency with one carrier wave in said first group of channels, but displaced in phase by 90 electrical degrees with respect to said carrier wave in said first group of channels, a demodulating system for each channel comprising an oscillator for furnishing a demodulating carrier wave identical in phase and frequency with the carrier wave of said channel, a low pass filter, and a demodulator adapted to deliver to said filter a wave having an instantaneous amplitude equal to the linear product of the instantaneous amplitude of said demodulating carrier and the sum of the instantaneous amplitudes of the carrier waves of all channels, and said demodulator comprising a mixer tube and a biasing network connected together such that the biasing network causes operation of the mixer tube in that region of its characteristic where the signal grid-plate transductance is substantially constant with change in signal grid voltage.

6. A multi-channel transmission system comprising a transmitting station having a plurality of carrier frequency generators, a plurality of modulators modulating each carrier frequency with an audio signal, means for transmitting said plurality of modulated carrier signals to a receiver comprising, a plurality of frequency generating means generating the various frequencies of the incoming modulated signals, phase-controlling means receiving the outputs of said frequency generating means to synchronize them with the incoming carrier signals of the same frequency, a plurality of demodulators receiving the entire input of said receiver and an input from said phase-controlling means, and each demodulator linearly multiplying the inputs to obtain the audio output of the carrier frequency equal to the frequency of the signal received from the phasecontrolling means, and. said demodulators comprising mixer tubes biased so as to operate in that region of their characteristics where the signal grid plate transductance is substantially constant with change in signal grid voltage.

: DONALD B. HARRIS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,608,566, Potter Nov. 30, 1928 1,882,119 Chireix Oct. 11, 1932 1,885,010 Day Oct. 25, 1932 2,017,886 Barden Oct. 22, 1935 2,213,941 Peterson Sept. 3, 1940 2,256,317 Earp Sept. 16, 1941 2,258,439 Bennett Oct. 7, 1941 2,276,863 Peterson Mar. 17, 1942 2,323,250 Smith June 29, 1943 2,342,286 Kock Feb. 22, 1944 2,410,883 Larsen et a1. Nov. 12, 1946 FOREIGN PATENTS Number Country Date 15,559 Great Britain Dec. 13, 1933 

